1. Field of the Invention
The invention relates generally to radiation to optical image converters, and more particularly to ultrafast high sensitivity converters without scintillators or phosphors.
2. Description of the Prior Art
Research with short pulse high power lasers, inertial confinement fusion (icf), and free electron x-ray lasers has created a need for x-ray to optical converters and optical wavelength image converters with ps or sub ps time resolution, imaging capability, and high sensitivity. These converters can be used as the basis of framing cameras gated with short pulse laser beams, as the input for high-speed optical streak cameras, or as a means to record temporal variations of the x-ray or optical flux with chirped frequency coherent probe beams. In experiments that generate large pulsed neutron or gamma ray fluxes, such as icf research, the damaging effects of neutron and gamma induced backgrounds make the operation of conventional electro-optical detectors difficult or impossible around the area of the experiment. In these cases, high bandwidth conversion of detected signals into coherent optical beams allows for the transport of the optical signal to remote protected detectors where they can be safely recorded.
Currently time resolved x-ray to optical conversion or optical wavelength conversion is commonly performed using scintillators or phosphors, which produce UV or visible incoherent light. The fastest time response demonstrated with these materials is a sub 25 ps rise time and 160 ps decay time in ZnO:Ga phosphors and crystals. These materials also have limited x-ray stopping power, low optical output power, and broad angular output.
Work has been done attempting to use carrier induced refractive index changes in semiconductors to modulate probe beams in quantum wells and bulk semiconductors through high angle scattering, interferometric effects in Fabry Perot etalon structures, or surface reflectivity modulation. Scattering at large angles caused by individual x-ray photons is typically weaker than the surface scattering from even the best polished surfaces at low intensities, and theoretically vanishes at higher intensities because the x-ray induced refractive index variation becomes spatially uniform. Probe beam intensity can be modulated in amplitude with uniform irradiation in Fabry Perot etalon structures, but they must be operated with small throughput to get good signal contrast and sensitivity. Lack of parallelism and scattering in the thick etalons needed for good x-ray absorption and sensitivity limits the contrast and sensitivity achievable in practice. Etalon time response for picosecond pulsed optical probes degrades as the finesse and thickness of the etalon increase, and etalons lose sensitivity as the line width of the probe laser approaches a fraction of the width of the resonance. High finesse etalons are extremely sensitive to variations in etalon thickness much smaller than the wavelength of the probe light, which makes accurate image conversion difficult. Reflectivity modulation has low sensitivity because the reflectivity changes very little and very slowly as a function of absorbed radiation flux, and the probe samples the semiconductor material only within an optical skin depth of the surface.
It is desirable to provide a radiation to optical converter for optically recording x-ray or optical signals with continuous and gated picosecond time resolution, high sensitivity, and good signal contrast.